The manufacture of integrated circuits includes processing steps resulting in the formation of geometrical patterns of material on a substrate. An example of such a pattern is a photoresist mask formed on the substrate by a photolithographic process. These patterns typically comprise a plurality of very narrow strips or lines of material disposed on a substrate.
There has been a constantly increasing demand for smaller electronic equipment which, of course, requires smaller electronic circuitry, particularly, very small integrated circuits. This has resulted in a need for ever smaller structures to be fabricated on substrates, such as semiconductor wafers, so that more circuitry can be packed into a given space. In recent years, as a result of the desire for smaller integrated circuits, manufacturing techniques have been developed which are capable of producing structures on substrates having dimensions of less than 1 micron, for example, lines of material that are less than one micron wide. The dimensions of these structures are critical parameters which must be monitored and controlled at various stages of the manufacturing process if very small, high density integrated circuits are to be successfully produced.
In the past, line width measurement in integrated circuits has been performed using standard optical microscopes. However, the resolution of such microscopes is limited by the wavelength of the light employed and the aperture of the objective lens in the microscope. Because of the wavelength of visible light and the nature of objective lenses, it is difficult to reliably measure dimensions smaller than about one micron. The resolution of such microscopes has been extended somewhat by using shorter wavelength light, such as blue or ultraviolet light, or by using lenses with extremely high numerical apertures, such as water-immersion lenses. An increase in resolution of about 1.4 times may be achieved by using the confocal optical scanning microscope concept. This concept is referred to in an article by C. J. R. Sheppard and A. Choudhury, entitled "Image formation in the scanning microscope", appearing in Optica Acta, 1977, Vol. 24, No. 10, pp. 1051-1073. There is, however, a limit to what may be achieved in terms of resolution with optical microscopes. The limitations are such that even the techniques for extending resolution do not result in sufficient accuracy for measuring the very narrow line widths required in many of today's integrated circuits.
To overcome the limitations of measurements made with optical microscopes, scanning electron microscopes have been developed which will accurately measure the width of the narrowest line capable of being made using present day integrated circuit manufacturing technology. However, the expense and complexity of such electron microscopes makes their use impractical in an integrated circuit production environment. Specifically, these electron microscopes make it difficult to achieve high throughput in a production environment. Thus, they have been used only to calibrate optical measuring apparatus capable of such high throughput.
Another technique for measuring line widths is described in Kleinknecht et al. U.S. Pat. Nos. 4,200,396 and 4,303,341. See also Kleinknecht et al. U.S. Pat. Nos. 4,039,370, 4,141,780, 4,330,213, and 4,408,884. The technique involves indirectly measuring line width by observing the characteristics of a diffraction grating formed on a test surface adjacent an integrated circuit pattern, both the grating and the circuit pattern being formed on the same semiconductor wafer. In this technique, material is simultaneously removed from the circuit area of the semiconductor wafer and the adjacent test surface to form an integrated circuit pattern and a diffraction grating on the same wafer. The average width of a series of strips in the diffraction grating is measured by exposing the entire diffraction grating to a beam of monochromatic light. The diffraction grating diffracts the light beam into a plurality of beams of different orders. The intensities of the first and second order diffracted beams, referred to by the patents as I1 and I2, are measured by photodetectors placed at appropriate diffraction angles. The average width of the strips making up the entire diffraction grating is calculated as a function of the ratio I2/I1. This average width of the strips in the diffraction grating is then taken as the width of the lines in the integrated circuit pattern adjacent the diffraction grating.
Although this technique is capable of optically determining the width of lines disposed on substrates in an integrated circuit manufacturing process, it does not directly measure the actual width of any of those lines in the integrated circuit pattern. The accuracy of line width values inferred from the average strip width in the diffraction grating accordingly may be limited for very small line widths.
As a consequence of the difficulty of accurately measuring small line widths, a long felt but unfulfilled need has existed for an apparatus and method which will directly and accurately measure the dimensions of an object disposed on a substrate, such as the width of a line in an integrated circuit pattern, which is practical in a commercial integrated circuit manufacturing environment.